Conventionally, in radio communication using the CDMA method, a transmit signal is spread and transmitted, and on the receiving side a received signal is despread using the same spreading code as on the transmitting side. By this means, in CDMA radio communication, signals directed to a plurality of users can be code division multiplexed and transmitted by preparing a plurality of mutually orthogonal spreading codes.
Also, in CDMA communication, when a transmission error is detected on the receiving side, the transmitting side is requested to retransmit the same signal, and thus the error rate characteristics of receive data are improved.
FIG. 1 shows the configuration of a conventional CDMA transmitting apparatus. CDMA transmitting apparatus 1 sends a modulated transmit signal to a spreading section 3 via a control section 2. The spread signal obtained by means of spreading section 3 is transmitted via a radio transmitting section (RF) 4 that performs radio transmission processing such as digital-analog conversion processing and signal amplification, and an antenna AN.
CDMA transmitting apparatus 1 also inputs a CDMA signal received by antenna AN to a despreading section 7 via a radio receiving section (RF) 6 that performs radio reception processing such as analog-digital conversion processing. The signal despread by despreading section 7 is output as a received signal, and is also sent to a retransmission request detection section 8. Retransmission request detection section 8 detects a retransmission request signal included in the received signal, and sends the detection result to control section 2.
Control section 2 is provided with a buffer, and if there is a retransmission request, the previously transmitted transmit signal stored in the buffer is output as a retransmission signal. Control section 2 also performs control of retransmission signal transmission timing and so forth.
However, with a conventional CDMA radio transmitting apparatus, particularly when channel fluctuations are slow, errors may occur consecutively even when a signal directed to a specific user is retransmitted. In this case, the problem of an excessive increase of the number of retransmissions arises. Since transmission delay increases as the number of retransmissions increases, the major problem of decreased transmission efficiency occurs. There is thus a method whereby retransmissions are discontinued when a given time is reached, but in this case a new problem arises of degradation of error rate characteristics.